In the recovery process of a pulp mill, green liquor is formed partly by dissolving and partly by dilution of the smelt obtained in the recovery boiler. The green liquor is an aqueous solution of sodium sulfides, sulfates and carbonate, The sodium carbonate in the green liquor is causticized to sodium hydroxide (white liquor) and lime mud by adding burnt lime, which sodium hydroxide is used in the cooking process.
Several separation stages are used in the processing of green liquor and white liquor. A typical set up includes a first green liquor separation unit for removing dregs in the green liquor as well as a separation unit for drying dregs, thereafter follows a white liquor filter after complete causticizing where lime mud is filtered out, obtaining a clear white liquor. The lime mud is finally washed and dewatered in order to displace residual alkali and obtain a lime mud at high dry matter content which is fed to the lime kiln in order to convert it to burnt lime, which subsequently is reintroduced into the causticizing process. The separation units used in these process positions, mainly filters, could be of different kinds, i.e. disc filters, drum filters, sedimentation vessels, centrifuges or others and they could either be pressurized or atmospheric.
For final lime mud dewatering it is necessary to obtain maximum dry matter content of the lime mud and low residual alkali and sulfur content, as TRS (Totally Reduced Sulfur) emission regulations applicable for the lime kiln are strict. The residual alkali in lime mud is proportional to the amount of remaining sulfur dioxide, which is discharged in the kiln as TRS. As environmental concerns are constantly in focus; lower emissions for continued operations of the mill will be an important issue.
In order to reduce residual alkali and sulfur content in lime mud, it becomes a natural approach to add yet another wash step for the lime mud. This approach may not be feasible in all mills as available free land area for yet another washing machine is not at hand. Another approach is to integrate multiple wash stages in one wash machine. One such concept is disclosed in U.S. Pat. No. 8,002,994 where an atmospheric lime mud disc filter is designed as a 2-stage washing and thickening stage, having more filter discs in the last thickening stage. A disadvantage with this 2-stage machine is that it is complicated to operate as stages are in sequence and directly ahead of the lime kiln which must be fed with an even flow of dry lime mud in order not to cause disturbances in operation. If the lime kiln needs shutting down, it may take hours to regain stable operation, and the integrity of the lime kiln could be at risk. A solution to solve these problems with an uninterrupted feed of dry lime mud to the lime kiln is shown in WO2011/078749 where instead the lime mud filter is designed as 2 stages in parallel. By parallel operations of 2 stages, one stage can always be in operation, while the other may call for a clean-out process, which may require a considerable down time of this stage.
Another problem for the filters used in the recovery process is the residual alkali content in slurries, including the content of thiosulfate, which is corrosive and dictates what material must be used in these filters. The first white liquor filter handles slurries with high alkali content and must use expensive high-alloy stainless steel, while the lime mud thickening stage may use less expensive low-alloy stainless steel. In the first lime mud wash after the white liquor filter, considerable levels of residual alkali could still be at hand. If some disturbance occurs in the operations of the white liquor filter, residual alkali levels could increase abruptly and during a limited time expose the next filter stage as well to high alkali levels.
In FIG. 1 is shown the principal process stages of a recovery process in the causticizing plant. Smelt S from the recovery boiler is the input flow to the process and the produced white liquor WL is the output flow from the process. The smelt is added to a smelt dissolver SD during addition of weak white liquor WW from a storage tank STWW, and the formed solution is called green liquor and is sent to an equalizing tank EQGL. From this equalizing tank it is pumped to a separation process, here a clarifying vessel CLGL, where dregs and particles settle in the bottom. The dregs are forwarded to a dregs storage tank STD before being forwarded to dregs washing, here dregs filter FD. The dregs is washed out by adding hot water HW and scraped off from the dregs filter to a dregs container D, and is most often sent to landfill or other destruction. The filtrate from the dregs filter FD contains washed out alkali at low concentration and is sent to the weak white liquor storage tank STWW. The clarified green liquor is pumped from the clarifying vessel CLGL to a green liquor storage tank STGL. The temperature of the green liquor is often close to the boiling temperature of the green liquor, i.e. somewhat above 100° C., and before being fed to the slaker SL it needs cooling by a cooler LC, as the following causticizing process is exothermic in nature and boiling is not wanted. The cooled green liquor is thus added to the slaker together with addition of burnt lime from a storage bin STBL. The causticizing process starts in the slaker during separation of grits, i.e. not dissolved lime particles and other particles, discarded to a grit container G. The completion of the causticizing process is done in the causticizing train, i.e. a number of causticizing vessels in series, CT1->CT2->CT3. After completion of the causticizing process the process liquor KGL typically contains 90% by weight of white liquor with an effective alkali content (NaOH) of about 100-120 g/l and 10% by weight of lime mud. The causticized process liquor KGL is therefore sent to the pressurized white liquor filter FWL, where a clear white liquor is obtained and sent to a storage tank STWL for strong white liquor. The gas phase of this filter is pressurized by mill air PMA and subsequently recirculated through the filter shaft to a separator and through compressor C to create a filtration force over the filter elements. A lime mud cake is formed on the filter elements and washed with hot water HW, which reduce residual alkali levels in the lime mud cake. The lime mud cake obtained in the white liquor filter FWL, is diluted and sent to a storage tank STLM for lime mud. In order to drain or wash out more residual alkali in the lime mud, the lime mud is sent to a lime mud filter FLM. The filtrate side of the lime mud filter FLM is set under vacuum by evacuation pump P connected to the gas phase of the weak liquor separation tank on the filtrate side. In this lime mud filter the main objective is to increase dry matter content of the lime mud before it is sent to the lime kiln LK; typically the dry matter content is in the range 70-80%, but as indicated in FIG. 1 this lime mud filter may also perform some washing by addition of hot water HW. The dried lime mud LM is directly sent to the lime kiln LK in order to convert it to burnt lime BL which is returned to the process and the storage bin STBL. The filtrate from the lime mud filter FLM is sent to the weak white liquor storage tank STWW.
In FIG. 2 is the principle process stages of FIG. 1 modified by using a 2-stage lime mud filter reassembling the type of filter disclosed in U.S. Pat. No. 8,002,994. In the first stage FLM1 of this lime mud filter, lime mud is dewatered and washed, subsequently the lime mud cake from this stage is reslurried in a tank RS while adding dilution water, and this lime mud slurry is fed to second stage FLM2 where it is thickened to appropriate dryness.